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CMS-EXO-22-017 ; CERN-EP-2024-022

Search for long-lived heavy neutral leptons decaying in the CMS muon detectors in proton-proton collisions at $ \sqrt{s}= $ 13 TeV

CMS Collaboration

28 February 2024

Phys. Rev. D 110 (2024) 012004

Abstract: A search for heavy neutral leptons (HNLs) decaying in the CMS muon system is presented. A data sample is used corresponding to an integrated luminosity of 138 fb$ ^{-1} $ of proton-proton collisions at $ \sqrt{s}= $ 13 TeV, recorded at the CERN LHC in 2016-2018. Decay products of long-lived HNLs could interact with the shielding materials in the CMS muon system and create hadronic and electromagnetic showers detected in the muon chambers. This distinctive signature provides a unique handle to search for HNLs with masses below 4 GeV and proper decay lengths of the order of meters. The signature is sensitive to HNL couplings to all three generations of leptons. Candidate events are required to contain a prompt electron or muon originating from a vertex on the beam axis and a displaced shower in the muon chambers. No significant deviations from the standard model background expectation are observed. In the electron (muon) channel, the most stringent limits to date are set for HNLs in the mass range of 2.1-3.0 (1.9-3.3) GeV, reaching mixing matrix element squared values as low as 8.6 (4.6) $\times$ 10$^{-6} $.

Links: e-print arXiv:2402.18658 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures
png pdf	Figure 1: Feynman diagrams for the production of a Majorana HNL $ N_M $ (left) and a Dirac HNL $ \overline{N_D} $ (right) via a $ \mathrm{W^-} $ boson decay and through its mixing with an SM neutrino of the same flavor. The prompt lepton from the $ \mathrm{W^-} $ boson serves as a clean signature for triggering, whereas the decay products of the HNL are reconstructed as a cluster of muon detector hits.
png pdf	Figure 1-a: Feynman diagrams for the production of a Majorana HNL $ N_M $ (left) and a Dirac HNL $ \overline{N_D} $ (right) via a $ \mathrm{W^-} $ boson decay and through its mixing with an SM neutrino of the same flavor. The prompt lepton from the $ \mathrm{W^-} $ boson serves as a clean signature for triggering, whereas the decay products of the HNL are reconstructed as a cluster of muon detector hits.
png pdf	Figure 1-b: Feynman diagrams for the production of a Majorana HNL $ N_M $ (left) and a Dirac HNL $ \overline{N_D} $ (right) via a $ \mathrm{W^-} $ boson decay and through its mixing with an SM neutrino of the same flavor. The prompt lepton from the $ \mathrm{W^-} $ boson serves as a clean signature for triggering, whereas the decay products of the HNL are reconstructed as a cluster of muon detector hits.
png pdf	Figure 2: Distribution of $ N_{\text{hits}} $ (upper) and $ \Delta\phi_\text{lep} $ (lower) for DT clusters (left) and CSC clusters (right). Signal distributions of a Majorana HNL with $ m_{\mathrm{N}} = $ 2 GeV and $ c\tau_{0}= $ 1 m are compared with the OOT background distributions selected with $ t_\mathrm{cluster}^\mathrm{DT} $ matched to bunch crossings earlier than the PV for DT clusters and $ t_\mathrm{cluster}^\mathrm{CSC} < - $12.5 ns for CSC clusters. The centroids of the clusters in the signal events are required to be within $ \Delta R= $ 0.4 of the HNL's direction. The distributions are normalized to unit area. The shapes of the distributions shown are similar for the electron and muon channels.
png pdf	Figure 3: Definition of the ABCD plane. The area of the blue squares illustrates the relative amount of expected events in each of the bins, with bins B and C having the majority of the event yields. Bin D is the signal region.
png pdf	Figure 4: Comparison of $ N_{\text{hits}} $ distributions for events with muons from $ \mathrm{Z}\to\mu\mu $ between data and simulation, for CSC clusters (left) and DT clusters (right), using data collected in 2017 and the simulation of the corresponding data-taking conditions. The data sample is selected by requiring a two-muon invariant mass consistent with a Z boson and one of the muons is matched to an MDS cluster. Data-to-simulation correction factors are applied to the $ \mathrm{Z}\to\mu\mu $ simulation. Only statistical uncertainties are included in the figure. Distributions made using data collected in 2016 and 2018 are found to have similar shape as the 2017 data sample.
png pdf	Figure 4-a: Comparison of $ N_{\text{hits}} $ distributions for events with muons from $ \mathrm{Z}\to\mu\mu $ between data and simulation, for CSC clusters (left) and DT clusters (right), using data collected in 2017 and the simulation of the corresponding data-taking conditions. The data sample is selected by requiring a two-muon invariant mass consistent with a Z boson and one of the muons is matched to an MDS cluster. Data-to-simulation correction factors are applied to the $ \mathrm{Z}\to\mu\mu $ simulation. Only statistical uncertainties are included in the figure. Distributions made using data collected in 2016 and 2018 are found to have similar shape as the 2017 data sample.
png pdf	Figure 4-b: Comparison of $ N_{\text{hits}} $ distributions for events with muons from $ \mathrm{Z}\to\mu\mu $ between data and simulation, for CSC clusters (left) and DT clusters (right), using data collected in 2017 and the simulation of the corresponding data-taking conditions. The data sample is selected by requiring a two-muon invariant mass consistent with a Z boson and one of the muons is matched to an MDS cluster. Data-to-simulation correction factors are applied to the $ \mathrm{Z}\to\mu\mu $ simulation. Only statistical uncertainties are included in the figure. Distributions made using data collected in 2016 and 2018 are found to have similar shape as the 2017 data sample.
png pdf	Figure 5: The expected and observed number of events in the signal region (bin D) of different event categories. Signal yields of a 2 GeV Majorana HNL with the mean proper decay length of 1 m are added to the expected background.
png pdf	Figure 6: Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The $ \tau $ neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure.
png pdf	Figure 6-a: Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The $ \tau $ neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure.
png pdf	Figure 6-b: Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The $ \tau $ neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure.
png pdf	Figure 6-c: Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The $ \tau $ neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure.
png pdf	Figure 6-d: Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The $ \tau $ neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure.
png pdf	Figure 6-e: Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The $ \tau $ neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure.
png pdf	Figure 6-f: Expected and observed upper 95% CL limits on $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $ (upper), $ |V_{{\mathrm{N}} \mu}|^2 $ (middle) and $ |V_{{\mathrm{N}} \tau}|^2 $ (lower) as functions of the HNL mass ($ m_{\mathrm{N}} $) for a Majorana (left) and Dirac (right) type HNL. The $ \tau $ neutrino mixing limit is obtained by combining the results from the electron and muon channels. For these limit calculations, the HNL is assumed to mix with a single lepton flavor state only. The differences between the expected and observed limits on $ |V_{{\mathrm{N}} \mu}|^2 $ are not visible in this figure.
png pdf	Figure 7: The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV, respectively.
png pdf	Figure 7-a: The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV, respectively.
png pdf	Figure 7-b: The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV, respectively.
png pdf	Figure 7-c: The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV, respectively.
png pdf	Figure 7-d: The largest values of the Majorana (upper) and Dirac (lower) HNL mass (left) and mean proper decay length (right) parameters that are excluded at 95% CL are shown as a function of the mixing matrix elements squared ratios $ f_\ell $ with the three lepton generations, considering a mean proper decay length of 1 m and a fixed mass of 1.5 GeV, respectively.

Tables
png pdf	Table 1: Validation of the ABCD method in the OOT and in-time validation regions. The predictions of the method for the signal bin (last column) are consistent with the observed number of events, shown in the second-to-last column.
png pdf	Table 2: The event yields in the bins A, B, and C are shown in each of the event categories considered in the search, as well as the prefit prediction for the ABCD background in the signal-enriched bin D.
png pdf	Table 3: Summary of the $ \mathrm{Z}\to\mu\mu $ background estimate in different categories. The first three columns show the estimates in the $ \mathrm{Z}\to\mu\mu $ enriched control region of the total background and its $ \mathrm{Z}\to\mu\mu $ and non-muon-induced components. The fourth column shows the transfer factors $ \zeta $ used to predict the $ \mathrm{Z}\to\mu\mu $ background in the signal region, shown in the fifth column.
png pdf	Table 4: Summary of systematic uncertainties affecting the signal yield prediction. For DT clusters, the systematic uncertainties due to jet and muon vetoes are found to be negligible and are omitted. The uncertainties are reported relative to their impact on the predicted signal yield.
png pdf	Table 5: Excluded ranges of $ |V_{{\mathrm{N}} \ell}|^2 $ for Majorana and Dirac type HNLs at select HNL masses. The chosen HNL masses are those at which the excluded values of $ |V_{{\mathrm{N}} \ell}|^2 $ have the smallest magnitude.

Summary

A search for long-lived Dirac or Majorana heavy neutral leptons (HNLs) has been performed using proton-proton collision data at $ \sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The search targets events with one prompt electron or muon and a muon detector shower (MDS) that would result from HNL decays occurring in the CMS muon detector. The presence of the MDS signature along with the associated vetoes and identification criteria suppresses the standard model background by a factor exceeding 10$^{7} $, while maintaining typical signal efficiencies of 25-35%. No significant excess over the standard model background is observed. The results are interpreted as 95% confidence level limits on the HNL mixing matrix elements squared $ |V_{{\mathrm{N}} \mathrm{e}}|^2 $, $ |V_{{\mathrm{N}} \mu}|^2 $, and $ |V_{{\mathrm{N}} \tau}|^2 $. We also present limits on the HNL mass and mean proper decay length as a function of the mixing matrix element squared fractions to the three lepton generations. The most stringent limits to date for HNLs in the mass range of 2.1-3.0 (1.9-3.3) GeV are set, reaching squared mixing matrix element values as low as 8.6 (4.6) $\times$ 10$^{-6} $ in the electron (muon) channel.

Additional Figures
png pdf	Additional Figure 1: Expected and observed upper 95% confidence level (CL) limits on Majorana HNL production as a function of the HNL mass ($ m_{ \mathrm{N}} $) and coupling strengths on pure muon coupling. Results from previous CMS searches [41,38], as well as other experiments, including ATLAS [35], BEBC [27], Belle [31], CHARM [28], DELPHI [29] and NuTeV [30], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments.
png pdf	Additional Figure 2: Expected and observed upper 95% confidence level (CL) limits on Dirac HNL production as a function of the HNL mass ($ m_{ \mathrm{N}} $) and coupling strengths on pure muon coupling. Results from previous CMS searches [41,38], as well as other experiments, including ATLAS [35], BEBC [27], Belle [31], CHARM [28], DELPHI [29] and NuTeV [30], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments.
png pdf	Additional Figure 3: Expected and observed upper 95% confidence level (CL) limits on Majorana HNL production as a function of the HNL mass ($ m_{ \mathrm{N}} $) and coupling strengths on pure electron coupling. Results from previous CMS searches [41,38], as well as other experiments, including ATLAS [35], BEBC [27], Belle [31], CHARM [28] and DELPHI [29], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments.
png pdf	Additional Figure 4: Expected and observed upper 95% confidence level (CL) limits on Dirac HNL production as a function of the HNL mass ($ m_{ \mathrm{N}} $) and coupling strengths on pure electron coupling. Results from previous CMS searches [41,38], as well as other experiments, including ATLAS [35], BEBC [27], Belle [31], CHARM [28] and DELPHI [29], are shown as reference. The limits from ATLAS, CMS and DELPHI experiments are set at 95% CL, and the other shown limits are set at 90% CL. The hatched side of the lines indicate regions excluded by the other experiments.

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